[Luis MC]

Hi all,

     I've been studying the output stage of buffamp preamplifier and I am asking me if a higher quiescent current would be better for linearity.

     Pspice show me an imbalance in AC currents for each output transistor, the upper transistor drives more AC signal current that the bottom one, and this might be bettered if we raising the values of resistors in NFB loop for lower AC current through output transistors.

     Might be an error in pspice too...

     Thanks,
     Luis Cárdaba

[davidw]

If there is an imbalance in the buffer amplifier it can only be caused by differences in the BD139/BD140 characteristics and/or asymmetry in the opamp. Increasing the value of the feedback resistors will cause an increase in noise but won't improve anything useful, like thd. You don't say how large the effect appears to be. The real buffamp measures very well but could probably be improved a little by using a darlington pair or cfp output stage.

David